Displays for augmented reality (AR) place higher demands on OLEDs (organic LEDs) than VR applications in terms of brightness and contrast. Through a clever combination of enhancements, Sony Semiconductor Solutions have tripled the brightness of OLED microdisplays while maintaining the same service life. As a result, these displays are now also suitable for the fast-growing AR market.
Recent years have brought great advances in organic light emitting diodes (OLEDs). OLEDs are luminous thin-film components made from organic semiconductors. They are finding their way into more and more new application areas, including near-to-eye (NTE). This includes electronic viewfinders (EVFs) as well as head-mounted displays (HMDs). The latter are used in virtual reality (VR) and augmented reality (AR) applications, as well as in systems that combine the user’s natural perceptions with artificial, computer-generated perceptions (mixed reality, MR).
Demanding NTE applications can in principle be implemented using a variety of technological approaches. OLEDs have been popular for EVFs and VR for some time – but until recently they lacked the brightness required for AR. One of the established methods for generating high- brightness images is Digital Light Processing (DLP). Developed by Texas Instruments, this projection technology uses a microelectromechanical system (MEMS) within the projection path. It generates an image by reflecting light off rapidly repositionable micromirrors. Another approach is to use LCDs (Liquid Crystal Displays) or LCoS (Liquid Crystal on Silicon). But even the latest LCoS models are not capable of reproducing completely dark black values, because of their inherent light-blocking display structure. Black tends to appear dark-grey, which makes these displays weak on contrast (see fig. 2).
Impressive advantages of OLEDs
In recent years, manufacturers like Sony Semiconductor Solutions have made significant enhancements to optimise OLED microdisplays for AR applications, where high brightness, good contrast and low power consumption are essential. OLED displays have been used generally in NTE applications for years. Now, with their dramatically improved brightness, the time is ripe for AR too. Today they are regarded as a real alternative to DLP and LCoS technology.
OLED microdisplays offer ultra-high resolution and high brightness with tiny display dimensions. Thanks to their low power consumption, small size and simplified control electronics, OLED microdisplays are ideally suited to small, power-saving embedded systems. Equipped with dedicated optics to magnify the tiny display surface, OLED microdisplays are really easy to integrate into NTE and projection applications.
OLED microdisplays generate images very quickly, so movement appears extremely fluid (see fig. 3). Due to the high resolution, views are super sharp and very high-contrast.
Comparison with LCoS technology
Conventional OLED displays have a response time in the microsecond range. Broadly speaking, they are over 1,000 times faster than LCoS displays. When it comes to contrast – the difference
between bright and dark areas of an image – modern OLED microdisplays achieve a ratio of 100,000:1, as opposed to 150:1 on LCoS displays.
OLEDs maintain high colour fidelity even at low grey levels. Their luminosity is achieved with low power consumption since no backlighting is required. Low temperatures do not cause problems, whereas LCoS may start to appear “sluggish” below -10 °C as their response time slows. OLEDs also have the advantage of high pixel density and high resolution. For these reasons, OLED displays are increasingly replacing currently widespread LCoS technology (see fig. 4).
Figure 4: OLED microdisplays are being used in more and more applications
Nevertheless, in terms of maximum achievable brightness, OLED screens are generally inferior to LCoS. OLED displays can also suffer from the well-known problem of “burn-in”, where adjacent pixels age and exhibit different brightness properties due to the same screen content being displayed repeatedly over long periods. This raises a question mark over their suitability e.g. for industrial applications with long product lifecycles. However, such problems are now eliminated in most applications thanks to the latest developments in OLED microdisplay technology. These include modern features like the Sony Semiconductor Solutions “orbit” function, but current OLEDs in general also have a longer lifespan and increased light outcoupling efficiency.
Challenges and solutions for EVFs and VR
Space of course is limited in EVFs and HMDs for VR applications. So developers value a small display form factor. To enlarge the field of view while retaining the same format, it makes sense to increase the resolution by decreasing the pixel pitch. Sony Semiconductor Solutions second-generation OLED microdisplays had a pixel pitch of 7.8 µm (3,300 ppi). Now the manufacturer has reduced this value to 6.3 µm. This means a pixel density of more than 4,000 ppi and therefore a higher resolution with the same form factor.
It was found that adjusting the emission structure using colour filters resulted in higher emission efficiency, which means longer life can be achieved. In the latest OLED manufacturing process, the colour filter is deposited directly on the silicon substrate. An on- chip colour filter (OCCF) array of this kind results in a smaller distance between the colour filter and the light-emitting layer. This greatly reduces the possibility of photon crosstalk. It also improves the viewing angle because the filter is now closer to the subpixel. This enlarges the radiation angle and hence the viewing angle (see fig. 5).
Figure 5: Comparison between OCCF design and conventional OLED structure
VR developers are taking advantage of these advances – and achieving very good results with the latest Sony OLED microdisplays. A typical 0.5-inch microdisplay for VR applications is the ECX339A. With UXGA resolution of 1,600 x 1,200 pixels, a frame rate of 120 fps and a contrast ratio of 100,000:1, it is ideally suited to VR applications. With a minimal pixel pitch of 6.3 µm, the small display has a high pixel density of 4,032 ppi. Its maximum brightness of 1,000 cd/m² is perfectly adequate for VR, but not optimal for AR. However, excellent alternatives are now available.
Challenges and solutions for AR
Especially in AR applications, it is crucial that the information layers displayed in wearables are not only rich in contrast, but also blend seamlessly into the real environment. OLED microdisplays have to be capable of displaying these superimposed images completely transparently with no colour breaking (see fig. 6). AR applications also require brightness of more than 1,000 cd/m² in the wearable. This means the OLED microdisplay has to have a nominal value of at least 3,000 cd/m², allowing for transmission losses during projection. Sony Semiconductor Solutions is therefore keen to further optimise microdisplay brightness.
Figure 6: OLED microdisplays blend the additional information layer seamlessly into the real environment (right)
The demand for higher brightness values has led to changes in the lithography process. One technique borrowed from sensors is to apply specially shaped (hemispherical) microlenses directly to the colour filter in the glass substrate. In the light outcoupling from each pixel, these microlenses focus the light so that it passes cleanly through the filter in its red, green or blue wavelength ranges, with no photon crosstalk. This increases brightness values by a factor of 1.8, and prevents colour distortion even with large viewing angle changes of +/-50° (see fig. 7).
The demand for higher brightness values has led to changes in the lithography process. One technique borrowed from sensors is to apply specially shaped (hemispherical) microlenses directly to the colour filter in the glass substrate. In the light outcoupling from each pixel, these microlenses focus the light so that it passes cleanly through the filter in its red, green or blue wavelength ranges, with no photon crosstalk. This increases brightness values by a factor of 1.8, and prevents colour distortion even with large viewing angle changes of +/-50° (see fig. 7).
Furthermore, by choosing a new cathode material, Sony Semiconductor Solutions has achieved significant improvements in light outcoupling. Instead of a magnesium-silver alloy, a highly transparent and very conductive zinc oxide alloy has proven effective. At a wavelength of around 450 nm, for example, this improves the outcoupled light intensity by a factor of 1.6.
Since the change of cathode material has different effects at different wavelengths, the transmission properties of the colour filters have to be adjusted accordingly. Across the whole spectrum, the change of cathode material achieves an overall improvement in light outcoupling by a factor of 1.3.
New OLED module paves the way for AR applications
One example of successful implementation of the described enhancements is the new Sony ECX335S OLED module. It has been available in mass production since 2020. Offering peak brightness values of up to 3,000 cd/m², it is especially suited to AR applications. The OLED microdisplay’s high contrast makes additional information layers appear seamless and ensures a “real” AR experience.
The ECX335S offers a currently unbeaten combination of Full HD resolution together with outstanding brightness of up to 3,000 cd/m². With an extremely small form factor and a contrast ratio of 100,000:1, this module will continue to spur innovative AR solutions. This example illustrates that OLED microdisplays are being used in NTE applications and increasingly also in demanding AR applications.
About the Author
David Kallenbach is a field application engineer and OLED expert at FRAMOS in Taufkirchen. After completing his master’s degree in physics, with a focus on nanostructures in semiconductors, he gained international experience as a technical sales engineer for industrial displays based on LCD, TFT and OLED technologies. He is one of the top OLED experts in the technical support team at FRAMOS and he knows how to work hand-in-hand with leading technology providers to meet customer requirements.
